U.S. Pat. No. 4,130,623 discloses a method of heating a sheet of heat shapeable plastic material, preferably PVC, ABS or polyolefin, to a heat shaping temperature which can rise to 350.degree. or 400.degree. F.=176.degree. or 204.degree. C., and of gripping it between a tool which bears marking contours and supporting means forming an elastomeric anvil having a hardness A of approximately 35 to 60, in order to produce on the sheet patterns consisting of contours and hollows. In the case of a PVC sheet, the heat shaping temperature is at least 280.degree. F.=137.degree. C., corresponding to a plastic or softened state.
Applicant has investigated whether it is possible to produce well controlled incised engravings on tubular pieces of plastic material without creating distortions, and, if possible, while avoiding the constraints of a heating operation.